1. Field of the Invention
The present invention relates to a golf ball having a layer composed of a cross-linked rubber.
2. Description of the Related Art
A flight distance is one of the important performances of the golf ball demanded by a golfer. The golf ball that flies a long distance refreshes the golfer and contributes to gaining good scores. It is necessary to improve the repulsive performance of the golf ball to improve the flight distance thereof.
Feeling is another important performance of the golf ball demanded by the golfer. The golf ball for which the golfer has a soft feeling gives the golfer a sense of security and contributes to the stability of swinging.
That is, it is important that the golf ball has a good repulsion performance and gives the golfer a soft feeling. To achieve this, various investigations have been made to improve the physical property of the golf ball. For example, in Japanese Patent Application Laid-Open No. 6-154357, there is disclosed a two-piece golf ball in which device is made in distribution of hardness. In Japanese Patent Application Laid-Open No. 8-243192, there is disclosed a two-piece golf ball in which the flexural rigidity modulus of the cover, the surface hardness of the core, and the sectional hardness of the core are set to a predetermined range, respectively. Further, in addition to the hardness of the golf ball, investigations have been made on other values of its physical property such as its compressive strains amount, and Young""s modulus and loss factor, both of which are measured by a viscoelastic spectrometer.
The repulsion performance and the golfer""s feeling are manifested in the state in which the golf ball is actually hit, namely, in a dynamic state. On the other hand, the above-described hardness, flexural rigidity modulus, compressive strain amount, Young""s modulus, and loss factor are so-called static physical properties of the golf ball. Thus, no matter how much the static physical properties are investigated, it is difficult to achieve the improvement of the dynamic performance sufficiently.
A repulsive viscoelasticity spectrometer can be used as a means for measuring the physical property of the golf ball in the dynamic state. The repulsive viscoelasticity spectrometer measures the physical property of a specimen by applying a dynamic strain thereto. However, the speed of the strain imparted to the material of the golf ball by the repulsive viscoelasticity spectrometer is as low as 0.001/sec to 0.1/sec, and a maximum strain is also as low as 0.01% to 2%. This is because the material of the golf ball has a high hardness and is thus not deformed greatly by a force applied thereto by the repulsive viscoelasticity spectrometer. On the other hand, when the golf ball is actually hit by a golfer, the speed of the strain of the golf ball lies in the range of 2000/sec to 5000/sec and its maximum strain is in the range as high as 5% to 25%. That is, the golf ball undergoes a high-speed and large deformation. There is a big difference between the degree of the strain of the golf ball in the case where a force is applied thereto by the repulsive viscoelasticity spectrometer and the degree of the strain thereof when it is actually hit. Therefore, the repulsive viscoelasticity spectrometer does not measure the dynamic property of the golf ball in a state similar to the state in which it is actually hit. To improve the repulsive performance of the golf ball and allow the golfer to have a soft feeling when it is hit, it is necessary to optimize the dynamic property of the golf ball in a state similar to the state in which the golf ball is actually hit.
The present invention has been made in view of the above-described situation. It is an object of the present invention to provide a golf ball having its dynamic property optimized in a state similar to the state in which the golf ball is actually hit and thus having a high repulsion performance and allowing a golfer to have a soft feeling.
To achieve the object, according to the present invention, there is provided a golf ball having a layer composed of a cross-linked rubber whose Young""s modulus lies in the range of 30 Mpa to 100 Mpa both inclusive and whose loss factor lies in the range of 0.01 to 0.45 both inclusive, when the Young""s modulus and the loss factor are measured by a split Hopkinson""s bar tester whose impact bar has a collision speed of 14.0 m/sec.
The golf ball of the present invention has the layer composed of the cross-linked rubber. The Young""s modulus of and loss factor of the layer composed of the cross-linked rubber lie within the predetermined range, respectively when they are measured by the split Hopkinson""s bar tester. As will be described later, in the measurement made by the split Hopkinson""s bar tester, a specimen undergoes a high-speed and a large strain. Accordingly, the viscoelastic characteristic value (Young""s modulus and loss factor) of the specimen can be measured in a state similar to the state in which a golf ball is actually hit. It is possible to enhance the dynamic performance of the golf ball, namely, to improve the repulsion performance and allow a golfer to have a soft feeling by optimizing the Young""s modulus and the loss factor.
The Young""s modulus of :the layer composed of the cross-linked rubber lies in the range of 30 Mpa to 100 Mpa both inclusive when the split Hopkinson""s bar tester measured it. Because the Young""s modulus lies within the range, the golf ball of the present invention has a preferable repulsion performance and allows a golfer to have a soft feeling when it is hit. That is, if the Young""s modulus is less than the lower limit of the range, the repulsion performance thereof may deteriorate, whereas if the Young""s modulus is more than the upper limit of the range, the golf ball may give the golfer a hard feeling when it is hit. From this point of view, it is preferable to set the Young""s modulus to lie in the range of 40 Mpa to 80 Mpa both inclusive.
The loss factor of the layer composed of. the cross-linked rubber lies in the range of 0.01 to 0.45 both inclusive when the split Hopkinson""s bar tester measured it. Because the loss factor lies in this range, the golf ball has a preferable repulsion performance and allows the golfer to have a soft feeling when it is hit. That is, if the loss factor of the layer composed of the cross-linked rubber is less than the lower limit of the range, the golf ball may give the golfer a hard feeling when it is hit, whereas if the loss factor thereof is more than the upper limit of the range, the repulsion performance of the golf ball may deteriorate. From this point of view, it is preferable to set the loss factor of the layer to lie in the range of 0.1 to 0.3 both inclusive.
The golf ball of the present invention may be a one-piece golf ball consisting of the layer of the cross-linked rubber satisfying the above-described conditions or may be a two-piece golf ball consisting of the core composed of the layer of the cross-linked rubber and the cover covering the core. The golf ball may be a multi-piece golf ball having three or more layers, one layer of which is composed of the cross-linked rubber. Above all, the two-piece golf ball is particularly favorable because it display""s the effect of the layer composed of the cross-linked rubber satisfying the above-described conditions.